Electron radiographic system with liquid absorber

ABSTRACT

An electron radiographic system for making x-ray pictures by producing an electrostatic image on a dielectric sheet suitable for development by xerographic techniques. A pair of electrodes with a liquid filled gap therebetween, with the liquid taking several forms. A dielectric sheet is positioned at an electrode and x-rays are directed past the object to the liquid for producing electrons and positive ions in the gap. The electrons and positive ions are attracted toward the respective electrodes for deposit on the dielectric sheet to produce the latent electrostatic image. The liquid is x-ray opaque and electrically nonconducting and may be a solvent with a gas or nonionic metallic compounds dissolved therein, or may be in liquefied gas.

United States Patent Allan et al.

[451 Mar. 25, 1975 ELECTRON RADIOGRAPHIC SYSTEM Primary Examiner-James W. Lawrence WITH LIQUID ABSORBER Assistant Examiner-B. C. Anderson [75] Inventors Frank V Aumr John H Lewis Attorney, Agent, or Firm-Harris, Kern, Wallcn &

Katherine J. Lewis; Arthur L. Tmsley Morsell, all of Los Angeles; Eric P. Muntz, Pasadena; Paul B. Scott, [57] ABSTRACT Topanga; Murray S..Welkwsky,

I An electron radiographic system for making x-ray pic- LOS Angeles of Cahf' tures by producing an electrostatic image on a dielec- [731 Assignee: Xonics, lnc., Van Nuys, Calif. tric sheet suitable for development by xerographic techniques. A pair of electrodes with a liquid filled [22] Filed 1974 gap therebetween, with the liquid taking several [21] Appl. No.: 456,532 forms. A dielectric sheet is positioned at an electrode and x-rays are directed past the object to the liquid for producing electrons and positive ions in the gap. The 56 1 34 82 electrons and positive ions are attracted toward the [58] g 492 335 respective electrodes for deposit on the dielectric i 5 5 1 sheet to produce the latent electrostatic image. The liquid is x-ray opaque and electrically nonconducting and may be a solvent with a gas or nonionic metallic [56] References Cited compounds dissolved therein, or may be in liquefied UNITED STATES PATENTS gas. 3,774,029 ll/l973 Mutz et al 250/315 12 Claims, 1 Drawing Figure :PL: 1 4? l6 s I -X 35 4* 6 39 9st l737 Z s5 ao gf t 3 Le g lf 3 9 w s '44 5 i a Q 7 s "we This invention relates to electron radiographic systems of the type shown in U.S. Pat. No. 3,774,029 and in particular, to a new and improved system with a liquid adsorber in the gap between the electrodes in place of the conventional gas.

In an electron radiographic system, a dielectric receptor sheet is positioned at one of the electrode surfaces in a gap between a pair of electrodes. A source of x-rays is directed to the gap past the object being xrayed, and incoming x-ray photons generate electrons and positive ions in the gap for attraction toward the respective electrodes. Charges are collected on the dielectric receptor providing a latent electrostatic image of the object, and this image is developed to a visual image by standard xerographic techniques with the density of the deposited toner powder being a function of the magnitude of the electrostatic charge.

In the electron radiographic system as described in said US. patents, a radiopaque gas is maintained in the gap, typically of 8-15 mm width, at super atmospheric pressure, typically five to ten atmospheres. While the gas absorber systems provide highly satisfactory results, the production and maintenance of the high pressures in the gap result in problems in design, manufacture and operation of the imaging chambers or cassettes which hold dielectric receptor and gas during the x-ray exposure. Also, the 8-l5 mm gap required for the gas absorber system results in problems related to image resolution which necessitate the use of spherically shaped electrodes or a spherical electric field between the elecrodes as described in copending U.S. application, Ser. No. 388,212, filed Aug. 14, 1973.

The present invention is directed to a new and improved electron radiographic imaging system which may be operated at ambient pressure and with a gap less than mm and which utilizes a liquid as the x-ray absorber in place of the gas of the prior art devices.

One advantage to be derived from a liquid absorber is the requirement ofless gap thickness and, in the case of dissolved gases, less pressure for a desired quantum efficiency in the imaging system. Also, the substantially higher density of the liquid restricts the photoelectron range to a very small value essentially independent of pressure in the gap, with a resultant improvement in resolution.

Three forms for the radiopaque liquid x-ray absorber are contemplated in the present invention, comprising liquified gases, liquids whose molecular structure includes heavy atoms, and heavy atom radiopaque material dissolved in a liquid solvent. For the electron radiographic process utilizing liquid state absorbers, heavy atom is intended to mean atoms of atomic number 17 or greater. The limit of atomic number 17 is determined by the ratio of x-ray photons which are absorbed via the photoelectric process or scattered via the Compton collisional process within the imaging chamber. The electrons and positive ions produced by photoelectric absorption are correlated witih the target (body) absorption whereas the electrons and positive ions produced by an x-ray photon which has undergone a Compton collision (path deflection) are not correlated with the target absorption and thus tend to degrade the image quality. The ratio of photoelectric to Compton events for a typical x-ray photon of 50 KeV 2 is approximately one for an atom with atomic number 17. Since for atoms of lower atomic number the Compoton effect dominates, the useful limit for x-ray absorbers for imaging purposes is set at atomic number 17.

Accordingly, it is an object of the present invention to provide a new and improved elecron radiographic system utilizing a liquid x-ray absorber. Other objects,

advantages, features and results will more fully appear in the course of the following description. The drawing merely shows and the description merely describes preferred embodiments of the present invention which are given by way of illustration or example.

The single FIGURE of the drawing is a vertical sectional view of an x-ray imaging system employing a liquid absorber which may be a heavy atom liquid or a solvent with dissolved radiopaque material or a liquified gas.

An imaging system particularly adapted for making chest x-ray pictures is illustrated with an x-ray source 10 spaced from a receptor housing 11 and arranged so that the person or other object 12 to be x-rayed can be positioned between the x-ray source and the housing, preferably against the plate 13 of the housing.

A receptor supply roll 16, an imaging chamber 17, a liquid developer unit 18, and a toner fusing station 19 are provided within the housing 11. Hinged doors 20, 21 provide access to the interior of the housing. The dielectric receptor sheet 25 is fed from the roll 16 through the imaging chamber 17, the developer unit 18 and the fusing station 19 by drive rolls and idler rolls as desired, and leaves the housing at an exit slot 26. Of course other imaging chamber configurations, including those shown in the prior art may be utilized as desired.

The imaging chamber 17 includes a vessel 30 with a U shaped passage 31 positioned within a channel portion 32 of the housing 1 1, preferably with thermal insulation material 33 placed about the vessel 30. In the preferred embodiment, a refrigeration unit 34 is connected to the vessel 30 for maintaining the vessel and contents at a temperature below ambient. Electrodes 37, 38 are mounted in the vessel 30 in spaced relation, defining a gap 39 therebetween.

The plate 13 of the housing, the insulation material 33, and the wall of the vessel 30 between the x-ray source and the electrode 37 should be made of materials having low x-ray absorbtion. Suitable materials for the electrodes 37, 38 are discussed in the aforesaid U.S. Pat. No. 3,774,029. A high voltage supply 40 is connected acrosss the electrodes 37, 38 via switch 41 and leads 42, 43. The receptor sheet 25 may be held in position against the electrode 37 by rollers 44, 45.

A liquid 50 is placed in the passage 31 of the vessel 30, filling the gap 39.- This iquid is an x-ray opaque electrically nonconducting liquid and in the embodiment illustrated comprises hexane with xenon dissolved therein. Preferably, the housing 11 is made substantially airtight and is charged with xenon at atmospheric pressure to reduce the likelihood of contamination of the hexane solvent by atmospheric constituents. The amount of gas which can be dissolved into the solvent increases with a decrease in temperature of the solvent and it has been found that the amount of xenon which can be dissolved in hexane can be about doubled by maintaining the hexane at -20C. Alternatively, the system can be operated at room temperature, eliminatto the housing via line 53. Both reduced temperatureand increased pressure may be employed to increase the xenon concentration in the liquid.

The electron radiographic system with the liquid in the gap between the electrodes operates in the same manner as the system of the aforementioned patent. The incoming x-ray photons are absorbed by the liquid in the gap and electrons and positive ions are produced. Electrons are attached toward the anode and the positive ions toward the cathode. Charges build up on the dielectric receptor forming a latent electrostatic image of the object exposed to the radiation. After the exposure, the receptor is advanced through the developer and the fusing station to provide the visual image and the resultant picture may be used in the same manner as the conventional x-ray picture.

In all embodiments of the present invention, the liquid absorber in the gap should be electrically nonconducting and should have an electrical resistivity greater than 10" ohmcentimeters when in use. A lower resistivity for the liquid tends to discharge any electrostatic image in contact with the liquid within a few seconds, thus making retrieval of the image very difficult. The liquid should be x-ray opaque or radiopaque, that is, the liquid should absorb incoming x-ray photons and produce electrons and positive ions. Also, the absorbing atoms of the liquid should have an atomic number of at least l7 and preferably at least 35.

Krypton may be used in place of xenon for dissolving into the solvent. Other suitable solvents include lsopar- G, P-Xylene, Toluene, Mesitylene, n Heptane, and certain commerical products such as Kerosine, Amsco 123-15. and Ultrasene. All ofthe above liquids have an Oswalds coefficient at room temperature for solubility of xenon which is greater than two and all of them can be purified to the extent that their electrical resistivity is at least l ohm-centimeters. However, the stability of certain compounds when subjected to x-rays varies and the most desirable for use in liquid absorbers are those which are the most stable such as n-hexane, nheptane, and lsopar G.

In operation it is desirable to have a saturated solution, since the efficiency of the system is a function of the amount of material dissolved in the solvent. Therefore the number density of the primary x-ray absorbing atoms (heavy atoms) desirably is at least 10 absorbing atoms per cubic centimeter or liquid. Other materials suitable for dissolving in the solvent include Bromine, lodine, and nonionic metallic compounds having an atomic number of at least l7 and preferably 35 or higher. as mixtures of such compounds. Examples of suitable non-ionic metallic compounds are sandwich compounds such as ferrocene and ruthenocene and related compounds. The lanthanide series elements such as La and Ce from non-ionic complexes with chelating compounds with the general formula where M denotes the metal atom and R,, R denote groups such as CH C H etc. Tetravalent tin compounds such as SnCl, and Snl are also non-ionic. All useable materials including orgonometallic compounds of Pb for example, must form nonconducting solutions and be sufficiently soluable to attain the required concentration of absorbers in the solution.

In an alternative embodiment, the liquid in the gap between the electrodes may be a liquid the molecules of which include heavy atoms in their structure, with an atomic number of at least l7 and preferably with an atomic number of 30 to 36 and higher. Suitable liquids include CCl,, CCl Br, CH l- CHFl CCl;,l, CH- Brl, CH- Cl l, CH Rl, CH Cl 21, other compounds of similar molecular composition containing elements of atomic number 17 or greater, and mixtures thereof. These liquids should have proper electrical resistivity (i.e., at least 10 ohm-centimeters), be sufficiently stable under x-ray radiation to maintain their characteristics during use preferably for several weeks, and in order to obtain an adequate quantum efficiency have a mass attenuation coefficient, ap (cm'-/gm), greater than 10 for a 50 KeV x-ray photon.

The liquid with molecules comprised of heavy atoms may be used in the apparatus of the drawing by placing it in the passage 31 of the vessel 30. In this embodiment, the system is usually operated at ambient temperature and ambient pressure, and the refrigeration unit and insulation material may be omitted. The housing may be charged with an inert gas such as carbon dioxide to reduce the likelihood of contamination of the liquid.

Another embodiment is the use of liquified gases such as liquid xenon or krypton as the liquid absorber. ln this case, the imaging chamber would be operated at the liquification temperature (165K for xenon) and the receptor would pass out of the refrigerated gap to a warmer region charged with gaseous xenon or other gas for development.

We claim:

1. In a radiographic system for operation with a ward said cathode for deposit of one of said types of charged particles on said dielectric sheet;

with the liquid in the gap moderating the energy of and reducing the mean free path of said electrons in the liquid to increase the quantity of and reduce the disperson of electrons moving through the liquid toward said anode and positive ions toward said cathode normal to said dielectric sheet.

2. A system as defined in claim 1 wherein said liquid 8. A system as defined in claim 1 wherein said liquid 7 comprises liquefied gas having an atomic number of at is a liquid with molecules including atoms of atomic number at least 17.

3. A system as defined in claim 1 wherein said liquid is a liquid with molecules including heavy atoms and selected from the gourp consisting of CCL, CClgBr, CH l CHFI CCl l, CH Brl, CH ClI, CH Rl, CHClgl, and mixtures thereof.

4. A system as defined in claim 1 wherein said liquid comprises a nonconducting solvent having dissolved therein, a non-ionic material having atoms with an atomic number of at least 17.

5. A system as defined in claim 1 wherein said liquid comprises a nonconducting solvent having dissolved therein, a material selected from the group consisting of xenon. krypton, bromine, iodine, non ionic metallic compounds having an atomic number of at least l7, and mixtures thereof.

6. A system as defined in claim 5 including means for maintaining said liquid at a temperature below ambient.

7. A system as defined in claim 5 including means for maintaining said liquid at a pressure greater than ambient.

least 36.

9. A system as defined in claim 8 including means for maintaining said liquid at a temperature below the boiling point of said gas.

10. A system as defined in claim 1 wherein said liquid has a resistivity of at least about 10 ohm-centimeters.

11. A method of producing an electrostatic image on a dielectric sheet, including the steps of:

positioning the dielectric sheet at an electrode in a gap between anode and cathode electrodes positioned adjacent an object to be imaged;

passing x-rays through said object and one of said electrodes;

absorbing incoming x-rays in the gap by maintaining in the gap an x-ray opaque electrically nonconducting liquid having atoms with an atomic number at least 17 and generating electrons and positive ions in the liquid, the liquid in the gap moderating the energy of and reducing the mean free path of said electrons to increase the quantity of and reduce the dispersion of electrons moving toward said anode and positive ions toward said cathode normal to said dielectric sheet; and

attracting electrons toward the anode and positive ions toward the cathode by applying a high potential across the electrodes depositing one of the types of charged particles on the dielectirc sheet.

12. The method of Claim 11 wherein said liquid is a liquid with molecules including atoms of atomic num- 

1. In a radiographic system for operation with a source of Xrays, the combination of: a pair of electrodes comprising an anode and a cathode; first means for supporting said electrodes in spaced relation with a small gap therebetween; a dielectric sheet in said gap at one of said electrodes; an X-ray absorber and electron and positive ion emitter in said gap between said anode and cathode for producing a charge image on said dielectric sheet, said emitter comprising an X-ray opaque electrically non-conducting liquid having atoms with an atomic number of at least 17; and second means for connecting a high voltage electric power supply across said electrodes for attracting electrons toward said anode and positive ions toward said cathode for deposit of one of said types of charged particles on said dielectric sheet; with the liquid in the gap moderating the energy of and reducing the mean free path of said electrons in the liquid to increase the quantity of and reduce the disperson of electrons moving through the liquid toward said anode and positive ions toward said cathode normal to said dielectric sheet.
 2. A system as defined in claim 1 wherein said liquid is a liquid with molecules including atoms of atomic number at least
 17. 3. A system as defined in claim 1 wherein said liquid is a liquid with molecules including heavy atoms and selected from the gourp consisting of CCl4, CCl3Br, CH2I2, CHFI2, CCl3I, CH2BrI, CH2ClI, CH2RI, CHCl2I, and mixtures thereof.
 4. A system as defined in claim 1 wherein said liquid comprises a nonconducting solvent having dissolved therein, a non-ionic material having atoms with an atomic number of at least
 17. 5. A system as defined in claim 1 wherein said liquid comprises a nonconducting solvent having dissolved therein, a material selected from the group consisting of xenon, krypton, bromine, iodine, non-ionic metallic compounds having an atomic number of at least 17, and mixtures thereof.
 6. A system as defined in claim 5 including means for maintaining said liquid at a temperature below ambient.
 7. A system as defined in claim 5 including means for maintaining said liquid at a pressure greater than ambient.
 8. A system as defined in claim 1 wherein said liquid comprises liquefied gas having an atomic number of at least
 36. 9. A system as defined in claim 8 including means for maintaining said liquid at a temperature below the boiling point of said gas.
 10. A system as defined in claim 1 wherein said liquid has a resistivity of at least about 1013 ohm-centimeters.
 11. A method of producing an electrostatic image on a dielectric sheet, including the steps of: positioning the dielectric sheet at an electrode in a gap between anode and cathode electrodes positioned adjacent an object to be imaged; passing x-rays through said object and one of said electrodes; absorbing incoming x-rays in the gap by maintaining in the gap an x-ray opaque electrically nonconducting liquid having atoms with an atomic number at least 17 and generating electrons and positive ions in the liquid, the liquid in the gap moderating the energy of and reducing the mean free path of said electrons to increase the quantity of and reduce the dispersion of electrons moving toward said anode and positive ions toward said cathode normal to said dielectric sheet; and attracting electrons toward the anode and positive ions toward the cathode by applying a high potential across the electrodes depositing one of the types of charged particles on the dielectirc sheet.
 12. The method of claim 11 wherein said liquid is a liquid with molecules including atoms of atomic number at least
 17. 